Conventional wear particle detectors capture metal particles which are present in the fluid of a hydraulic system, or more particularly, in the lubricant of an engine or transmission. These metal particles are induced to bridge a gap between two electrodes of such a device and thereby complete an electric circuit. Upon closing of the electric gap by the metal particles, an external warning signal is activated.
In systems whose moving components are primarily of steel, the effectiveness of the device is increased by a magnetic field between or in the direct proximity of the electrodes. In some systems, more than one pair of electrodes is used in order to increase the probability of metal particles closing the electric circuit. These ancillary provisions have no direct influence on the principle of this invention.
It is obvious that the same effect may result from the bridging of the electric gap by one metal particle of substantial size or by several metal particles of small size(s) in contact with the electrodes and with each other.
In particular, during normal operation of the engine, transmission or other hydraulic or fluid system, very fine particles are generated by the normal wear of power transmitting surfaces. This condition is particularly prevalent when the equipment is new and the power transmitting surfaces of gears, bearings and other components are in the initial wear process, commonly referred to as the run-in period. An accumulation of these very small particles, usually termed "wear fuzz," can cause an activation of the warning signal, even though no hazard exists, amounting to a so-called nuisance indication. If such devices are employed in engines, transmissions, gear boxes and auxiliary drives of, especially, aircraft, costly and unwarranted precautionary measures may result.
It would, therefore, be desirable to limit the activation of the warning signal to those cases where a metal particle of a larger size, such as produced by initial failure of a metal surface, bridges the gap between the electrodes of the device.